For the past week the Bishop of Limburg, Franz-Peter Tebartz-van Elst, has generated damaging headlines after the cost of his extravagant new residence complex blew out to at least 31 million euros (S$52.05 million).

The 53-year-old has been summoned to the Vatican for a decision on his fate, and was set to meet on Thursday with humility advocate Pope Francis, who has called for a "poor Church for the poor".

With many faithful in the German diocese in open revolt over the lavish bishop's palace and its 15,000-euro bathtub, there has been a sharp increase in local registered Catholics formally leaving the Church.

If the exodus in the ancient town of Limburg were echoed on a national scale, observers say it would be a severe blow to the Church - especially in a country where the faithful provide the bulk of its funding.

"I've never seen such a wave" of people quitting the faith, Ruediger Eschoffen, an official in charge of Church registrations in Limburg, was quoted as telling the newspaper Frankfurter Neue Presse.

Germany has a religion tax, and taxpayers must declare whether they are Catholic, Protestant or non-religious. People are however free to opt out and stop paying the so-called "Church tax".

Last year 295 Catholics in Limburg, in the central state of Hesse, officially left the Church, said Eschoffen. But he said the figure shot up to 29 people in a single day on Monday, after 18 left on Friday and 20 on Thursday.